1. Field of the Invention
This invention relates to ground anchor installations for resisting uplift loads. These ground anchors may be used, for example, to resist test loads which are applied to piles which have been driven into the ground near the ground anchors.
2. Description of the Prior Art
The following United States patents show ground anchors for resisting uplift loads: U.S. Pat. No. 1,081,654, U.S. Pat. No. 1,982,963, U.S. Pat. No. 3,012,644, U.S. Pat. No. 3,115,226 and U.S. Pat. No. 3,824,748. Each of these patents shows an elongated tension member which is either driven into the earth or is lowered down through a predrilled hole in the earth. The lower end of the tension member is provided with hinged flutes or arms which lie flat against the tension member to minimize resistance as it is lowered or driven into the earth. When an uplift load is applied to the tension member the flutes or arms pivot to flare out from the tension member and dig into the surrounding earth. In this manner uplift resistance is developed in the soil at the base of the tension member. U.S. Pat. No. 3,115,226 shows, in one embodiment, an alternate arrangement wherein the lower end of the tension member is formed to an undulated configuration and is positioned in an enlargement which is filled with concrete.
The uplift resistance capability of ground anchors corresponds to the weight of an inverted cone of earth extending up from the lower end of the anchor to the earth's surface and to the internal shear strength of the soil applied along the plane surface of this cone. It will be seen from this that the uplift resistance increases with greater ground anchor depths since greater anchor depths provide earth cones of greater size, weight and surface area. Another way in which uplift resistance can be increased without increasing the depth of the anchor is to provide a large diameter anchor base. This also provides a larger, though truncated, earth cone with increased weight and lateral surface area. The enlarged concrete base shown as one of the examples in U.S. Pat. No. 3,155,226 utilizes this principle.
It has been found that when an enlarged concrete base or bell is employed at the bottom of a cast-in-place concrete anchor caisson, the full lateral extent of the base cannot be counted on to establish the lower diameter of the truncated earth cone which provides the uplift resistance. This is because the concrete base, which is poured down into the enlarged bottom of the anchor hole alongside the tension member, is unreinforced; and an upwardly flaring shear plane projects through the concrete base or bell from the lower end of the tension member to the upper surface of the enlarged base. The effective lateral diameter of the concrete base therefore is the diameter of the base where that shear plane intersects the upper surface of the bell.
There is frequently a need, such as when testing piles in compression using uplift piles or caissons as a reaction, to establish uplift resistance entirely at some depth below the test pile, so as not to influence the test results. To develop this uplift resistance by friction alone would require exceedingly long piles or drilled shafts. Although one may employ a bell bottom drilled shaft so as principally to utilize the weight of the soil above the bell, unless the bell is reinforced one encounters the problems described above.